The relapse stage often happens when you think it is OK to have one drink (or one cigarette, etc) and you fall off the wagon. You might go back to precontemplation or you might go back to maintaining the changes you made.
The relapse stage is sometimes included in the stages of change model, in recognition that a person might have some or even many small lapses, or even relapses -- periods when the addictive behavior is taken up again -- before maintenance is achieved. In reality, the outcome of the process of change is highly individual -- some people are able to adjust to controlled drinking, drug use or addictive behaviors without becoming addicted. For others, abstinence is the only way that a person can keep their addiction under control. Sometimes it is only after several relapses that the person discovers what recovery from an addiction means for them.